**8. C-X/N/S/O…1, 2-diiodoterfluorobenzene systems**

One of the most commonly used XB donor is 1,4-diiodoterfluorobenzene due to its strong positive σ-hole on the iodine atom.[28,32,33] Similarly, 1,3-diiodoterfluorobenzene has been widely used as the Lewis acid in halogen bonds.[22,34,35] In the present study, however, we focus first on the XB systems with the less commonly used isomer of diiodotetrafluorobenzene, i.e., 1,2-diiodoterfluorobenzene (1,2-TFIB).[33,34,36–39] We will use 1,2-TFIB as the "probe donor" to study the halogen bonding interactions with different type of XB acceptors and to elucidate how the halogen bonding acceptor affects the properties of halogen bonding.

Cauliez. et al, used thiocyanate anion as the XB donor to construct co-crystals with 1,2-TFIB (Fig. 5).[34] Halogen bonds were observed between the neutral iodinated species (XB donor) and both the S and the N end of the thiocyanate anions, demonstrating the bidentate nature of SCN- . Both C-I…N and C-I…S presented strong linearity, and relatively strong halogen bonding interactions. On the other hand, the C-S…I and C-N…I angles follow roughly the directions of the free electron pairs on the acceptor atoms, providing an example of how the electronic structure of the XB acceptor affects the overall geometry of the molecular assembly. In this particular structure, weaker C-I…F interactions were also observed. In these contacts the fluorine atoms of the halogenated benzene acted primarily as XB acceptors. In addition to the XBs mentioned above, series of F…π, I…π, π…π, and F…H hydrogen bond contacts contributed to the crystal structure of the co-crystals of thiocyanate and 1,2-TFIB.

**Figure 5.** The halogen bond contacts in the co-crystal of the thiocyanate anion and 1,2-TFIB.[34]

A zigzag chain structure have also been obtained through C-I…N, and C-I…S halogen bonds in co-crystals of thiomorpholine (TMO) and 1,2-TFIB (Fig. 6).[33] Like thiosyanate, the thiomorpholine is able to act as a bidentate N, S halogen bond acceptor.[33,34] The cocrystal of TMO and 1,2-TFIB have been obtained by a simple mechanocemical synthesis i.e. by grinding the components together. The interesting feature of this stepwise cocrystallization process is that it is proposed to be guided by the competition of the strong and weak halogen bonds. The initially formed finite molecular assemblies are held together mainly by the stronger N…I bonds. These intermediates are then polymerized into infinite chains by cross-linking through weaker S…I interactions.

**Figure 6.** The zigzag chain of TMO/1,2-TFIB co-crystals.[33]

148 Recent Advances in Crystallography

properties of halogen bonding.

bidentate nature of SCN-

thiocyanate and 1,2-TFIB.

widely used as the Lewis acid in halogen bonds.[22,34,35] In the present study, however, we focus first on the XB systems with the less commonly used isomer of diiodotetrafluorobenzene, i.e., 1,2-diiodoterfluorobenzene (1,2-TFIB).[33,34,36–39] We will use 1,2-TFIB as the "probe donor" to study the halogen bonding interactions with different type of XB acceptors and to elucidate how the halogen bonding acceptor affects the

Cauliez. et al, used thiocyanate anion as the XB donor to construct co-crystals with 1,2-TFIB (Fig. 5).[34] Halogen bonds were observed between the neutral iodinated species (XB donor) and both the S and the N end of the thiocyanate anions, demonstrating the

strong halogen bonding interactions. On the other hand, the C-S…I and C-N…I angles follow roughly the directions of the free electron pairs on the acceptor atoms, providing an example of how the electronic structure of the XB acceptor affects the overall geometry of the molecular assembly. In this particular structure, weaker C-I…F interactions were also observed. In these contacts the fluorine atoms of the halogenated benzene acted primarily as XB acceptors. In addition to the XBs mentioned above, series of F…π, I…π, π…π, and F…H hydrogen bond contacts contributed to the crystal structure of the co-crystals of

**Figure 5.** The halogen bond contacts in the co-crystal of the thiocyanate anion and 1,2-TFIB.[34]

chains by cross-linking through weaker S…I interactions.

A zigzag chain structure have also been obtained through C-I…N, and C-I…S halogen bonds in co-crystals of thiomorpholine (TMO) and 1,2-TFIB (Fig. 6).[33] Like thiosyanate, the thiomorpholine is able to act as a bidentate N, S halogen bond acceptor.[33,34] The cocrystal of TMO and 1,2-TFIB have been obtained by a simple mechanocemical synthesis i.e. by grinding the components together. The interesting feature of this stepwise cocrystallization process is that it is proposed to be guided by the competition of the strong and weak halogen bonds. The initially formed finite molecular assemblies are held together mainly by the stronger N…I bonds. These intermediates are then polymerized into infinite

. Both C-I…N and C-I…S presented strong linearity, and relatively

Yet another zigzag chain structure has been obtained by co-crystallizing 1,2-TFIB with phenazine through C-I…N halogen bonds (Fig. 7).[36] Here the C-I…N halogen bonding is slightly weaker than that of the example shown in Fig. 6. This was most probably caused by the steric effect of the XB donor. It demonstrates that the overall geometry of the XB acceptor has an effect on the halogen bonding. The directionality of XB is clearly shown in the example on Fig. 7. The angle of the C-I…N is 169°, which is almost linear.

**Figure 7.** C-I...N halogen bonding in the zigzag chain structure of the co-crystal of 1, 2-TFIB with phenazine.[36]

The co-crystals of 4,4'-bipyridine and (1,2-TFIB) (Fig. 8) provide another example of C-I…N contacts. In this structure the C-I…N bonds linked the bipyridine (XB acceptor) and 1,2-TFIB together into two independent and almost perpendicular wave-like chains.[39]

**Figure 8.** Co-crystals 4,4'-bipyridine and (1,2-TFIB). The I…N distances range from 2.909 to 2.964 Å.[39]

The co-crystals of 2-mercapto-1-methylimidazole (mmim) and 1,2-TFIB show nicely the bonding preferences of XB (Fig. 9).[38] The mmim molecule and 1,2-TFIB form a complex, in which N-H…S bound imidazole dimers are connected through C-I…S interactions to a pair of 1,2-TFIB molecules, forming infinite chains. The C-I…S bonds are different in strength (the C-I…S distance of the weaker one was 3.843 Å, while the stronger one was 3.291 Å) and the bonds involving sulphur can be defined as trifurcated bonds. They consisted of two halogen bonds and one hydrogen bonds. The sulfur acts as the electron donor for both bonding types. In principle, the iodine could be also a hydrogen bond acceptor. However, it is solely devoted to the halogen bond, while the hydrogen bonds are formed only between the sulfur and NH of the imidazole ring.

**Figure 9.** The halogen and hydrogen bonds in the structure of (mmim)·(1,2-TFIB).[38]

As mentioned previously, oxygen can also be used as an XB acceptor to construct supramolecular structures. Co-crystals of the nitroxide 1,1,3,3-tetramethylisoindolin-2yloxyl (TMIO) and 1,2-TFIB are formed under standard sublimation conditions.[37] The formed 2:2 cyclic tetramer structure (Fig. 10), (TMIO)2·(1,2-TFIB)2, showed that each nitroxide oxygen atom, when serving as the XB acceptor, set up bifurcated halogen bonding with two iodine atoms from two 1,2-TFIB molecules, respectively. Again, the N-O…I angle follow the direction of the free electron pairs on the oxygen atoms, thus encouraging the tetrameric assembly of molecules. The O…I contacts in this motif were clearly shorter than the van der Waals contacts (down to 81.2%-83.1%), with strong directionality (C-I…O angles range from 170.30°-179.2°).

150 Recent Advances in Crystallography

and NH of the imidazole ring.

**Figure 8.** Co-crystals 4,4'-bipyridine and (1,2-TFIB). The I…N distances range from 2.909 to 2.964 Å.[39]

The co-crystals of 2-mercapto-1-methylimidazole (mmim) and 1,2-TFIB show nicely the bonding preferences of XB (Fig. 9).[38] The mmim molecule and 1,2-TFIB form a complex, in which N-H…S bound imidazole dimers are connected through C-I…S interactions to a pair of 1,2-TFIB molecules, forming infinite chains. The C-I…S bonds are different in strength (the C-I…S distance of the weaker one was 3.843 Å, while the stronger one was 3.291 Å) and the bonds involving sulphur can be defined as trifurcated bonds. They consisted of two halogen bonds and one hydrogen bonds. The sulfur acts as the electron donor for both bonding types. In principle, the iodine could be also a hydrogen bond acceptor. However, it is solely devoted to the halogen bond, while the hydrogen bonds are formed only between the sulfur

**Figure 9.** The halogen and hydrogen bonds in the structure of (mmim)·(1,2-TFIB).[38]

As mentioned previously, oxygen can also be used as an XB acceptor to construct supramolecular structures. Co-crystals of the nitroxide 1,1,3,3-tetramethylisoindolin-2-

**Figure 10.** The tetrameric unit (TMIO)2·(1,2-TFIB)2 (red = oxygen, purple iodine).[37]

As the examples above show, the 1,2-TFIB can be used as the XB donor with various acceptors. If, however, there are no other acceptors available, the "amphoteric" nature of 1,2-TFIB is revealed. When the 1,2-TFIB is crystallized from methanol, a structure with series of weak XB bonds can be obtained (Fig. 11).† The iodines form XB contacts, functioning as both the donors and the acceptors. The I…I distances are relatively long, ranging from 3.258 Å to 3.740 Å. Nevertheless, the distances are less than the sum of the van der Waals radii, and the directionality support the existence of the halogen bonds. The I…I contacts resulted in a zigzag structure that is further expanded through the F…F, F…I, and F…π halogen bonds. The F…F and F…I contacts are weak with long distances consisting of 2.783-2.924 Å and 3.258 Å for F…F and F…I, respectively. It should be noted that, to judge from the C-F…F angle (147.3°), the F…F contacts showed some amphoteric character. In the case of C-I…F, the fluorine atom behaves more clearly as the halogen bond acceptor, due to the existence of the negative lateral sides of the fluorine atom caused by the aspherical charge density distribution.[40]

**Figure 11.** Halogen bonding interactions in the crystal structure of 1,2-TFIB. †

## **9. X/Se…X system (X=Cl, Br, I)**

As early as in 1984, a study of Cl…Cl halogen bond in crystal structures of six dichlorophenols was carried out by Thomas and Desiraju.[41] This study was extended in 2011 by Mukherjee and Desiraju to 3,4,5-trichlorophenol and 2,3,4-trichlorophenol.[42] In the crystal structure of 3,4,5-trichlorophenol (Fig. 12), one Cl atom forms bifurcated halogen bonds with another chlorine and oxygen, respectively. This Cl, however, functions as XB acceptor with the other Cl, while at the same time, it serves as the XB donor for the oxygen atom, thus showing its dual nature.

**Figure 12.** Halogen and hydrogen bonding in the crystal structure of 3,4,5-trichlorophenol.[42]

Crystals with a zigzag sheet packing structure have been obtained by crystallizing 1-butyl-4, 5-dibromo-3-methylimidazolium iodide (Fig. 13).[43] The two bromine atoms of the cation act as the XB donors, while the iodine anion is the XB acceptor. The crystal structure shows that the iodide anions in the c-axis direction are positioned either at the top or the bottom of the zigzag structure, suggesting that the size of the halide anion has a strong effect on the zigzag sheet formation.[43]

152 Recent Advances in Crystallography

**Figure 11.** Halogen bonding interactions in the crystal structure of 1,2-TFIB. †

As early as in 1984, a study of Cl…Cl halogen bond in crystal structures of six dichlorophenols was carried out by Thomas and Desiraju.[41] This study was extended in 2011 by Mukherjee and Desiraju to 3,4,5-trichlorophenol and 2,3,4-trichlorophenol.[42] In the crystal structure of 3,4,5-trichlorophenol (Fig. 12), one Cl atom forms bifurcated halogen bonds with another chlorine and oxygen, respectively. This Cl, however, functions as XB acceptor with the other Cl, while at the same time, it serves as the XB donor for the oxygen

**Figure 12.** Halogen and hydrogen bonding in the crystal structure of 3,4,5-trichlorophenol.[42]

**9. X/Se…X system (X=Cl, Br, I)** 

atom, thus showing its dual nature.

**Figure 13.** Halogen bonding interactions in crystal structure of 1-butyl-4,5-dibromo-3 methylimidazolium iodide.[43]

The structure of co-crystal of 1,2-diiodoimidazole with 1,3,4-triiodoimidazole (Fig. 14) was studied by our group.‡ In this structure the two 1,2-diiodoimidzol molecules are linked by a I…I halogen bonding interaction (3.916Å), forming a dimeric unit. This unit is further connected with other 1,3,4-triiodoimidzole molecules, expanding the structure along crystallographic directions a, and b. The N-H…N hydrogen bonds expand the structure further in direction of c axis, constructing a 3D network. The bonding preferences of different interactions can be clearly seen in this structure. Halogen bonds are formed only between the iodine atoms, and each iodine atom of diiodoimidzol is trifurcated. In addition, it demonstrated the dual character of iodines, serving both as XB donors and as acceptors.

**Figure 14.** Hydrogen and halogen bonds in the co-crystal of 1,2-diiodoimidazol and 1,3,4 triiodoimidazol.‡

Halogen bonds involving selenium as the XB acceptor have not been widely studied.[29] The chain structure formed by connecting di-ter-butyliodophosphane selenide molecules through Se…I halogen bonding is one example of such a system.[29] In this structure the Se…I distance was found to be only slightly shorter than the sum of the van der Waals radii of selenium and iodine. Another example of Se…I interaction can be found in the crystal structure of iodoisopropylphosphane selenide.[29] In both structures, selenium is also involved in Se…H hydrogen bonds, providing another example of the interplay between the closely related electrostatic interactions.
